1. Field of the Invention
The present invention relates to a lathe comprising a headstock for rotatably supporting a main spindle for holding a workpiece, and first and second tool rests for holding tools, the first and second tool rests being movable in predetermined feed directions.
2. Description of the Prior Art
Conventionally, a machine tool having a structure shown in FIGS. 5 and 6 is known as a lathe. This lathe 100 comprises a bed 101 formed in the shape of a triangular prism becoming broad toward the bottom, and a first headstock 102, a second headstock 105, a first tool rest 110, a second tool rest 115 and the like provided on the bed 101 as shown in FIGS. 5 and 6 (refer to Japanese Unexamined Patent Publication No. 2002-59301).
The first and second headstocks 102 and 105 rotatably support main spindles 103 and 106 around the axes thereof, respectively, and are provided with rotation drive devices (not shown) for rotating the main spindles 103 and 106 around the axes thereof. The first and second headstocks 102 and 105 are mounted on the inclined face on the front side of the bed 101. In addition, the second headstock 105 is configured so as to move in the arrow-indicated Z-axis direction under the guidance of first guide rails 108 provided on the inclined face on the front side of the bed 101 in parallel with the axis lines of the main spindles 103 and 106. Furthermore, the main spindles 103 and 106 are provided with chucks 104 and 107 for holding workpieces.
The first tool rest 110 is provided between the first headstock 102 and the second headstock 105 and comprises a saddle 111 configured so as to move in the arrow-indicated Z-axis direction under the guidance of the first guide rails 108, and a tool rest body 112, configured so as to move in the arrow-indicated X-axis direction under the guidance of guide rails (not shown) provided on the saddle 111 orthogonally to the axis lines of the main spindles 103 and 106, for holding tools T.
The second tool rest 115 comprises a saddle 116 configured so as to move in the arrow-indicated Z-axis direction under the guidance of second guide rails 109 provided on the inclined face on the rear side of the bed 101 in parallel with the axis lines of the main spindles 103 and 106, a column 117 configured so as to move in the arrow-indicated Y-axis direction under the guidance of guide rails (not shown) provided on the saddle 116 orthogonally to the axis lines of the main spindles 103 and 106, a support base 118 configured so as to move in the arrow-indicated X-axis direction under the guidance of guide rails (not shown) provided on the column 117 orthogonally to both of the Z-axis and the Y-axis, a tool rest body 119, rotatably supported by the support base 118 in the arrow-indicated B-axis direction around a predetermined rotation center axis and configured so that indexing is made possible at a predetermined rotation angle position in the B-axis direction, for rotatably holding a tool T, and a drive motor 120, provided on the tool rest body 119, for rotating the tool T.
In this lathe 100, a workpiece held in the chuck 104 of the first headstock 102 or in the chuck 107 of the second headstock 105 is rotated together with the main spindle 103 or 106 by the rotation drive device (not shown). Furthermore, the first tool rest 110 or the second tool rest 115 in which the tool T is held is moved in a predetermined feed direction. Hence, a relative motion is carried out between the rotating workpiece and the tool T, whereby the workpiece is machined into a predetermined shape.
Machining is carried out in a state wherein the tool T of the second tool rest 115 has been fixed and held, in a state wherein the tool T has been rotated by the drive motor 120 or in a state wherein the tool T has been indexed at a predetermined rotation angle position by the rotation of the tool rest body 119 in the B-axis direction in accordance with machining requirements.
However, in the above-mentioned conventional lathe 100, the first headstock 102 and the second headstock 105 are integrally connected to the main spindles 103 and 106 and the rotation drive devices (not shown). Hence, for example, heat generated at bearings or the like for supporting the main spindles 103 and 106 is transferred to the various portions of the first and second headstocks 102 and 105 and the main spindles 103 and 106, whereby these are deformed thermally. As a result, an error occurs in the relative positional relationship between the workpiece held in the main spindle 103 and the tool T held in the first tool rest 110, and similarly an error occurs in the relative positional relationship between the workpiece held in the main spindle 106 and the tool T held in the second tool rest 115, whereby there is a problem of reducing the accuracy of machining.
In addition, for example, in the case when workpieces held in the chucks 104 and 107 are machined into a rectangular shape, a method shown in FIG. 7 is generally used. In other words, the tool T held in the tool rest body 119 is indexed at a predetermined angle position in the B-axis direction so that the axis line thereof is parallel with the axis lines of the main spindles 103 and 106 as shown in FIG. 7. Then, the tool T is moved in the Y-axis direction as shown in FIG. 7(a) to machine the workpiece W so as to obtain one side of the rectangular shape. Next, the workpiece W is rotated at 90-degree intervals as shown in FIG. 7 (b), (c) and (d) so as to be indexed, and the tool T is moved similarly in the Y-axis direction to machine the workpiece W so as to obtain the other sides of the rectangular shape.
However, conventionally, the accuracy of indexing the workpiece W is not always sufficiently high. Hence, the above-mentioned method has a problem wherein the squareness between the sides constituting the rectangular shape is not obtained at high accuracy.
In a general lathe, the orthogonal accuracy among the feed axes of the first tool rest 110 and the second tool rest 115 is usually set higher than the rotary indexing accuracy of the main spindles 103 and 106. Hence, in the case when the above-mentioned rectangular shape is obtained by machining, the squareness between the sides obtained by using a method wherein the tool T is moved along each side of the rectangular shape as shown in FIG. 8 can be made higher than the squareness between the sides obtained by using the above-mentioned method wherein the workpiece W is indexed at 90-degree intervals.
However, in the above-mentioned conventional lathe, the movement amount of the second tool rest 115 in the X-axis direction is limited by a certain amount because of the relationship with the first tool rest 100. Hence, the tool T cannot be moved beyond the axis lines of the main spindles 103 and 106 to the opposite side by a sufficient margin. Therefore, the machining method shown in FIG. 8 cannot be used. As a result, only the machining method shown in FIG. 7 can be used for the conventional lathe, whereby the above-mentioned problem cannot be solved.
In view of the above-mentioned circumstances, an object of the present invention is to provide a lathe capable of machining workpieces at high accuracy.